Method of manufacturing blockinglayer electrode systems



Patented Dec. 1, 1942 IVIETHOD OF MANUFACTURING BLOCKING- LAYERELECTRODE SYSTEMS Nicolaas Willem Hendrik Addink and Joseph AntoniusOtten, Eindhoven, Netherlands, assignors, by mesne assignments, toHartford National Bank and Trust Company, Hartford, Conn., as

trustee No Drawing. Application February 27, 1940, Serial No. 321,134.In Germany March 14, 1939 10 Claims.

This invention relates to a method of manufacturing blocking-layerelectrode systems of the kind comprising a selenium electrode and alsoto the blocking-layer electrode system produced thereby.

The invention has primarily for its object to improve the blocking layerof such an electrode system. The quality of this layer is of very highimportance as regards satisfactory operation of blocking-layer electrodesystems, for example in industrial rectifiers. As is well known, anincreased blocking efiect ensures that the leakage current is reduced sothat the losses are reduced,

and the efiiciency increases. In addition, this leakage currentprimarily governs the evolution of heat in the rectifier and it is thisevolution of heat which generally causes the electrode system todeteriorate. An increase of the blocking effect leads to a decrease ofth evolution of heat and thus to an increase of the life, and theloading capacity.

With blocking-layer electrode systems having a non-genetic blockinglayer, for example of polystyrol, it has been suggested to limit theleakage current by the provision of a bounding layer of high resistancebetween the blocking layer proper satisfactory blocking-layer effect.The inven-' tion is characterized in that the selenium layer has ablocking layer formed on it after its transformation, at least at thesurface, into the crystalline modification (Sen) exhibiting poorconductivity but before it is converted completely into the modification(Sec) of good conductivity.

It is known that selenium after being applied to a carrierand smoothed,generally shows the so-called amorphous structure, which isnon-conductive and is referred to hereinafter as SEA.

As is well V this selenium is subjected, if desired under a press, toheat-treatment, for example at about the amorphous selenium passes intothe crystalline modification which, however, is of poor conductivity andwhich is referred to herein as See. The transformation into theconducting modification, which is finally desired, takes place at asubstantially higher temperature, generally at about 200 C. The seleniumthus formed is referred to herein as See. Now, according to theinvention the blocking layer is not applied until crystalline selenium(Sea) has been formed at least superficially. Furthermore, the completetransformation of the crystalline selenium Sen into the conducting formsee does not take place until the blocking layer has been applied. Thus,the blocking layer is applied between two treatments the former of whichhas already led to the formation of See on the surface, whereas thelatter completes the process rendering the selenium conductive. It hasbeen found that this is particularly advantageous as regards thedensity, the uniformity and the firm adhesion of the blockin layer.

This method is particularly important in connection with that kind ofblocking-layer formation in which surface treatment of the selenium isconducted to render the substance on the surface (selenium and/or anadmixture therewith) non-conducting. Such a method is described inFrench specification 826,933, it being highly advantageous to carry outseveral successive forming treatments, each succeeding treatment beingpreferably performed at a higher temperature than the previous one. Asis well known, treatment at high temperature results in a continuoustransformation of the selenium because the time of applying the blockinglayer is chosen in such manner that the complete transformation into Seahas not yet taken place. On the other hand, a good basis for theblocking layer is provided because the crystalline selenium Sea hasalready been formed, at least superficially.

The selenium is found to be highly sensitive to the action of thesubstances that serve for the formation of the blocking layer during theprogess of transformation. If a second treatment is carried out at ahigher temperature than the first this progress in the transformation ofthe selenium is sure to ensue.

The complete building up of an electrode system according to theinvention will now be described and severalother measures that may beused with advantage will also be set out.

A quantity of molten selenium has added to it an admixture forincreasing the conductivity. A large number of such admixtures areserviceable. Thus, for example from 0.1 to 0.2%, for example 0.12%, ofzirconium chloride may be added.

A quantity of this mixture is applied in the'liquid state on arotaryaluminium carrier which may be previously roughened for betteradhesion and also provided with layers of zinc and carbon (see U. S.Patent No. 2,244,664, issued June 10, 1941). Due to the centrifugalforce the selenium is distributed evenly over the surface (see patentapplication Serial No. 254,508, filed February 3, 19 39). The seleniumlayer formed may have, for example, a

. thickness of 80 microns.

Next a small roughened mica plate is laid on the carrier comprising theselenium layer, said plate being previously coated on the side adjacentthe seleniuin with a liquid which, due to its action on the seleniumlayer, forms a blocking layer. Such liquids must have a comparativelyhigh boiling point because the action ensues at a temperature of about160 and the liquid must be prevented from being vaporised too rapidly.In addition it has been found that use may be made with advantage of asubstance in which the selenium is slightly dissolved. Due to theselenium particles on the surface being brought into solution a muchmore intense action-on the selenium and the admixture may result so thatit is more easily possible to render the material on the surfacenon-conductive. Satisfactory results are obtained by means of substanceshaving an alkaline reaction. Both the conversion of the zirconiumchloride into the zirconium oxide and an action on the selenium itselfmay be influential. The exact action on the selenium has, however, notyet been elucidated. As'an example of a substance that has the saidproperties we may mention quinoline.

The selenium layer and the mica plate, jointly with the interposedquinoline, are then compressed in a press having a temperature of about160. This treatment takes about 5 minutes. Due tothe compression theselenium layer is caused, to an even greater extent than that obtainedby centrifuging, to assume a uniform thickness and made compact. Due tothe action of the quinoline a firstblocking-layer fonnation also ensues.As may be seen from French Patent 826,933 no other treatment thanthisblockinglayer formation used to be performed and subsequenttreatment was only carried out for converting the S813 formed under thepress into the conducting modification Sec.

In accordance with the invention a further forming treatment is; carriedout with the selenium which is converted into the crystallinemodification (See) but has not yet obtained the final form Sec, sinceafter the aggregate has been withdrawn from the press and the mica plateis removed quinolin is again applied to the surface of the electrode.The quinoline is preferably applied by spraying, so that a finedistribution over the surface is obtained. The intimate contact betweenthe quinoline and the selenium surface which was obtained under thepress by the pressure of the press is now produced due to the fact thatthe quinoline is sprayed in fine drops on to the entire surface.

Secondly, this method of spraying or disintedisturbed.

If the blocking layer is to be further intensified this is not effectedby applying more quinoline, which involves a longer action, but ispreferably effected -by using a further independent forming treatment.

The second treatment described is carried out at a temperature betweenand the melting point of the selenium. The seleniumelectrode may, forexample, be introduced into a furnace having a temperature of 200 C.After the temperature of the plate has reached about the plate, whilestaying in the furnace, is subjected for 2 minutes to spraying withquinoline. The selenium electrode is then left in the furnace foranother three minutes.

To complete the construction of the electrode system the blocking layerthen has applied to it by spraying a complementary electrode consisting,for example, of a low-melting alloy of tin, bismuth and cadmium.

By splitting up the forming treatment in this manner and performing atleast one stage (in the example quoted the second stage) after Sea isformed and before the complete conversion into Sea has taken place it isensured that the blocking-layer formation takes place while the seleniumis in a condition which is particularly sensitive to the forming action.

Since this forming stage is independent of the application and smoothingof the selenium layer of the operation of rendering the selenium compactunder the press and finally of the final complete conversion into theconducting modification See itcan be controlled entirely and can beaffected in a manner capable of ready reproduc- The precedingblocking-layer formation under the press has the effect that a singlefollowing stage suflices in this case. Instead of carrying out theblocking-layer formation under the press it is, however, possible to adda blocking-layer forming treatment after compression.

Depending on theblocking layer desired to be formed the treatment may berepeated as many times as desired. I

Each successive treatment procures an intensification and greaterhomogeneity of the blocking layer obtained in the previous treatment.Feeble spots which lead to a high leakage current are in effectnullified.

The advantages obtained according to the invention areapparent from thetest results given below. The forward current of an electrode system ofwell-known form and also of a system in which blocking-layer formationhas been effected under the press by the action of an alkaline liquid,is, at 2 volts, about 0.25 amp./cm.. The admissible blocking voltage forsuch a system is from about 16 to 18 volts. In this case aleakage'current of about 8 maJcm. occurs: a hibglher leakage current isregarded as inadmis- S er Q In a system made by the method of theinvention the admissible blocking voltage is, however, from 30 to 45volts. It is most surprising that under these conditions the sameforward current can be obtained so long as care is taken that theconductivity of the selenium is sumciently high. This is ensured in theembodiment i described by the addition of the percentage of zirconiumchloride indicated. In spite of the high conductivity thus obtained aleakage current of only 8 maJcm. arises at the said blocking voltage.

From this it can be concluded that very high advantages can be obtained.Thus, for example, it was hitherto requisite to connect in series such anumber of electrode systems that the blocking test. Four small platesmade by known methods.

may be loaded in a Gratz circuit with an alternating voltage of 16volts. In this case 0.05 amp/cm. can be obtained at a direct outputvoltage of about 11.5 volts. If cooling plates are used in this case theoutput current can be increased to 0.1 ampJcmF.

When using electrode systems made according to the invention andcomprising cooling plates it is possible to apply an alternating voltageof volts in the same circuit. In this case it is also possible to obtain0.1 amp./cm. but at a direct output voltage of 23 volts. The energyobtained with the use of a corresponding number of elec-' trode systemsof corresponding size is consequently doubled.

In all these cases the operating temperature of the rectifiers iscomprised between and C.

What we claim is:

1'. In the manufacture of a blocking-layer electrode system, the stepsof applying a selenium electrode to a substratum, forming ablockinglayer on the selenium electrode after the selenium at thesurface thereof has been converted into its poor-conducting crystallinemodification (See), and converting the selenium at the surface of theelectrode into its good-conducting modifications (Sec).

2. In the manufacture of a blocking-layer elec-,

trode system, the steps of applying a selenium electrode to asubstratum, forming on the selenium electrode a blocking layer in aplurality of consecutive intensifying treatments after the selenium atthe surface thereof has been converted into its poor-conductingcrystalline modification (Sen), and converting the selenium at thesurface of the electrode into its good-conducting modification (Sec).

3. In the manufacture of a blocking-layer electrode system, the steps ofapplying a selenium electrode to a substratum, forming on the seleniumelectrode a blocking layer in a plurality of consecutive treatmentsafter the selenium at the surface of the selenium electrode has beenconverted into its poor-conducting modification (Sen), and convertingthe selenium at the surface of the electrode into its good-conductingmodification (Sec) the first treatment being effected at a lowertemperature than the subsequent treatments.

treatments after the selenium at least at the sur-' 4. In themanufacture of a blocking-layer electrode system, the steps of applyinga selenium electrode to a substratum, pressing the selenium electrode,forming 'a blocking-layer on the selenium electrode in a plurality oftreatments after the selenium at the surface of the selenium electrodehas been converted into its poor-conducting crystalline modification(See), and converting the selenium at the surface of the electrode intoits good-conducting modification (Sec) one of said blocking-layerforming treatments being effected simultaneously with the pressingstep.-- I I 5. .In the manufacture of a blocking-layer electrode system,the steps of applying a selenium ele trode to a substratum, forming'ablocking.- laye on the selenium electrode in a plurality of face'of theselenium electrode has been converted into its poor-conductingcrystalline modification (Sea) and before it has been converted into itsgood-conducting modification (See), and

heating the system simultaneously with one of the latter of saidblocking-layer forming treatments to convert the poor-conductingcrystalline selenium (See) into the good-conducting crystallinemodification (Sec).

6. In the manufacture of a blocking-layer electrode system, the steps ofapplying a selenium electrode to a substratum, forming a-blockinglayeron the selenium electrode by applying thereto a dosed quantity of asubstance which renders the surface of the selenium electrodenon-conductive, the formation of the blocking layer being eflectedafte'rat least the selenium at the surface has been convertedinto itspoorconducting crystalline modifications (S613), and

converting the selenium at the surface of the electrode into itsgood-conducting modifications (Sec).

7. In the manufacture of a blocking-layer electrode system, the steps ofapplying a selenium electrode to a substratum, forming a; blockinglayeron the selenium electrode by applying to the surface of the seleniumelectrode a dosed quantity of a substance which has an alkaline reactionand dissolves selenium, the blocking layer formation being effectedafter at least the selenium at the surface has been converted into itspoor-conducting crystalline modification (Sen), and converting theselenium at the surface of the electrode into its good-conductingmodifications (Sec).

8. In the manufacture of a. blocking-layer electrode system, the stepsof applying a selenium electrode to a substratum, forming a blockinglayer in a plurality of treatments, the first treatment being efi'ectedat a temperature of about C. after the selenium at the surface has beenconverted into its poor-conducting crystalline modification (Sen), andconverting the selenium at the surface of the electrode into itsgood-conducting modification (Sec), one of the latter of saidblocking-layer formation treatments being effected at a temperature ofabout 200 C.

9. In the manufacture of a blocking-layer selenium electrode system, thesteps of forming a selenium electrode on a carrier plate, placing apressing plate upon the surface of the selenium electrode with theinterposition of a uniformlydistributed layer of a liquid adapted to actupon the selenium to form a blocking layer, pressing the assembly whileheating the same to a temperature of about 160 C., removing the pressin;plate, applying to the so-formed surface a 'substance in afinely-divided state and adapted to act on the selenium to forma'blockinx layer, and heating the assembly at a temperature of about 200C.

10. A blocking-layer electrode system comprising a. selenium electrode,the selenium at the surface of the electrode consisting of thegoodconducting modification (Bee), and a blocking-

